ASoC: wsa881x: Add support for unloading speaker module

Invalidate debugfs_wsa881x_dent after it is
removed by a WSA device to avoid multiple
remove calls on debugfs_wsa881x_dent.

CRs-fixed: 2039099
Change-Id: Ifa9f633f49fff562e29290b2d1c1b976eeebd5ad
Signed-off-by: Karthikeyan Mani <kmani@codeaurora.org>